Pulses of optical signals used in optical wavelength-division multiplexing systems experience chromatic dispersion under the effect of optical fibers serving as optical transmission lines. Accordingly, a configuration is known in which optical relay stations and optical receiving stations include dispersion compensators for compensating for chromatic dispersion of optical pulses. Dispersion compensators differ in the amount of dispersion compensation required, depending on the lengths of optical transmission lines up to optical relay stations and optical receiving stations. Differences in the amount of dispersion compensation appear as propagation delays in dispersion compensators. That is, dispersion compensators provided in optical relay stations and optical receiving stations have different propagation delays.
Optical relay stations and optical receiving stations in optical wavelength-division multiplexing systems also include optical amplifiers for amplifying light attenuated through optical transmission lines. Optical amplifiers are known to have different response control timings of optical amplification due to differences in the wavelengths of pump lasers for pumping optical gain media in the optical amplifiers. In particular, a 980 nm pump laser, which is capable of optical amplification with superior noise characteristics, has a relaxation time in which excited electrons responsible for pumping fall to an energy level where they cause stimulated emission. Accordingly, if a variation occurs in the number of optical wavelengths used in an optical wavelength-division multiplexing system, gain adjustment in the optical gain medium lags behind the variation in the optical power input to the optical amplifier. This causes a problem in that a decreased number of optical signal wavelengths results in an excessively high gain per optical signal wavelength.
As a measure against the relaxation time of an optical amplifier, Japanese Laid-open Patent Publication No. 2002-261364 discusses that received light is delayed through a delay line before being input to an optical amplifying unit.